U.S. patent number 6,350,016 [Application Number 09/245,043] was granted by the patent office on 2002-02-26 for liquid ejecting method and liquid ejecting head.
This patent grant is currently assigned to Canon Kabushiki Kaisha. Invention is credited to Shuichi Murakami, Masayoshi Tachihara, Yasuyuki Tamura.
United States Patent |
6,350,016 |
Tachihara , et al. |
February 26, 2002 |
Liquid ejecting method and liquid ejecting head
Abstract
A liquid ejecting method using a liquid ejecting head having
electrothermal transducer elements for generating thermal energy
sufficient to create bubbles in liquid and ejection outlets
disposed opposed to the electrothermal transducer elements which
are arranged at a density not less than 300 per 25.4 mm in a line,
the liquid ejection head also having liquid flow paths in fluid
communication with the ejection outlets, respectively, wherein the
bubble generated by the thermal energy generated by the
electrothermal transducer element is brought into communication
with ambience while an internal pressure of the bubble is less than
an ambient pressure, and wherein droplets having volumes not more
than 15.times.10.sup.-15 m.sup.3 are ejected at a frequency not
less than 7 kHz, said method includes the improvement wherein the
liquid flow path of the liquid ejecting head has a height not less
than 6 .mu.m, and a distance between an upper surface and a lower
surface of the ejection outlet is not more than one half of a
minimum opening distance through a center of the ejection
outlet.
Inventors: |
Tachihara; Masayoshi (Chofu,
JP), Tamura; Yasuyuki (Yokohama, JP),
Murakami; Shuichi (Kawasaki, JP) |
Assignee: |
Canon Kabushiki Kaisha (Tokyo,
JP)
|
Family
ID: |
12260689 |
Appl.
No.: |
09/245,043 |
Filed: |
February 5, 1999 |
Foreign Application Priority Data
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Feb 10, 1998 [JP] |
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10-028879 |
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Current U.S.
Class: |
347/56;
347/61 |
Current CPC
Class: |
B41J
2/14024 (20130101); B41J 2/1404 (20130101); B41J
2002/14169 (20130101); B41J 2002/14387 (20130101) |
Current International
Class: |
B41J
2/14 (20060101); B41J 002/05 () |
Field of
Search: |
;347/65,54,56,57,44,47,20,61,45 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 641 654 |
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Mar 1995 |
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EP |
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195 05 465 |
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Aug 1995 |
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EP |
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0 925 930 |
|
Jun 1999 |
|
EP |
|
79-056847 |
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May 1979 |
|
JP |
|
84-123670 |
|
Jul 1984 |
|
JP |
|
84-138461 |
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Aug 1984 |
|
JP |
|
85-071260 |
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Apr 1985 |
|
JP |
|
4-10940 |
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Jan 1992 |
|
JP |
|
4-10941 |
|
Jan 1992 |
|
JP |
|
4-10942 |
|
Jan 1992 |
|
JP |
|
5-016365 |
|
Jan 1993 |
|
JP |
|
Primary Examiner: Barlow; John
Assistant Examiner: Mouttet; Blaise
Attorney, Agent or Firm: Fitzpatrick Cella Harper &
Scinto
Claims
What is claimed is:
1. A liquid ejecting method using a liquid ejecting head, said
method comprising:
generating thermal energy sufficient to create bubbles in liquid
using electrothermal transducer elements,
providing ejection outlets disposed opposed to the electrothermal
transducer elements, the ejection outlets being arranged at a
density not less than 300 per 25.4 mm in a line,
providing liquid flow paths in fluid communication with the
ejection outlets,
bringing the bubbles generated by the thermal energy generated by
the electrothermal transducer element into communication with
ambience while an internal pressure of each of the bubbles is less
than an ambient pressure,
ejecting droplets having volumes not more than 15.times.10.sup.-15
m.sup.3 at a frequency not less than 7 kHz, wherein
the liquid flow paths each have a height not less than 6 .mu.m,
and
a distance between an upper surface and a lower surface of each of
the ejection outlets is not more than one half of a minimum opening
distance through a center of each of the ejection outlets.
2. A method according to claim 1, wherein a sum of the distance
between the upper surface and the lower surface of the ejection
outlet and the height of the liquid flow path is not more than a
size of the electrothermal transducer element.
3. A method according to claim 1, wherein a volume of the droplet
is not more than 10.times.10.sup.-15 m.sup.3.
4. A method according to claim 1, wherein the height of the liquid
flow path is not more than 20 .mu.m.
5. A method according to claim 1, wherein an ejection speed of the
ejected droplet is not more than 20 m/s.
6. A method according to claim 1, wherein a width of an electrical
pulse applied to said electrothermal transducer element to eject
the liquid is not more than 3.5 .mu.sec.
7. A method according to claim 1, wherein a driving voltage of an
electrical pulse applied to the electrothermal transducer element
is in a range of 1.1 times to 1.3 times a threshold voltage of
liquid droplet ejection.
8. A method according to claim 7, wherein said electrical pulse
comprises a plurality of pulses.
9. A method according to claim 8, wherein said plurality of pulses
include a main pulse and a pre-pulse applied before the main pulse,
and a duration of the pre-pulse is not more than 1.5 .mu.sec.
10. A method according to claim 9, wherein an interval between said
pre-pulse and said main pulse is not more than 2.0 .mu.sec.
11. A method according to claim 1, wherein the liquid to be ejected
has a surface tension not less than 0.025 N/m and a viscosity not
more than 5.times.10.sup.-1 poise.
12. A method according to claim 1, wherein said electrothermal
transducer element generates enough thermal energy to cause film
boiling of the liquid.
13. A liquid ejecting head comprising:
electrothermal transducer elements for generating thermal energy
sufficient to create bubbles in liquid,
ejection outlets disposed opposed to the electrothermal transducer
elements, the ejection outlets being arranged at a density not less
than 300 per 25.4 mm in a line, and
liquid flow paths in fluid communication with the ejection
outlets,
wherein the bubbles generated by the thermal energy are brought
into communication with ambience while an internal pressure of the
bubbles is less than an ambient pressure,
droplets having volumes not more than 15.times.10.sup.-15 m.sup.3
are ejected from the ejection outlets at a frequency not less than
7 kHz,
the liquid flow paths each have a height not less than 6 .mu.m,
and
a distance between an upper surface and a lower surface of each of
the ejection outlets is not more than one half of a minimum opening
distance through a center of each of the ejection outlets.
14. A head according to claim 13, wherein a sum of the distance
between the upper surface and the lower surface of the ejection
outlet and the height of the liquid flow path is not more than a
size of the electrothermal transducer element.
15. A head according to claim 13, wherein a volume of the droplet
is not more than 10.times.10.sup.-15 m.sup.3.
16. A head according to claim 13, wherein the height of the liquid
flow path is not more than 20 .mu.m.
17. A head according to claim 13, wherein an ejection speed of the
ejected droplet is not more than 20 m/s.
18. A head according to claim 13, wherein a width of an electrical
pulse applied to said electrothermal transducer element to eject
the liquid is not more than 3.5 .mu.sec.
19. A head according to claim 13, wherein a driving voltage of an
electrical pulse applied to the electrothermal transducer element
is in a range of 1.1 times to 1.3 times a threshold voltage of
liquid droplet ejection.
20. A head according to claim 19, wherein said electrical pulse
comprises a plurality of pulses.
21. A method according to claim 20, wherein said plurality of
pulses include a main pulse and a pre-pulse applied before the main
pulse, and a duration of the pre-pulse is not more than 1.5
.mu.sec.
22. A method according to claim 21, wherein an interval between
said pre-pulse and said main pulse is not more than 2.0
.mu.sec.
23. A method according to claim 13, wherein the liquid to be
ejected has a surface tension not less than 0.025 N/m and a
viscosity not more than 5.times.10.sup.-1 poise.
24. A method according to claim 13, wherein said electrothermal
transducer element generates enough thermal energy to cause film
boiling of the liquid.
25. A liquid ejecting method using a liquid ejecting head having
electrothermal transducer elements for generating thermal energy
sufficient to create bubbles in liquid and ejection outlets
disposed opposed to the electrothermal transducer elements which
are arranged at a density not less than 300 per 25.4 mm in a line,
the liquid ejection head also having liquid flow paths in fluid
communication with the ejection outlets, respectively, wherein the
bubble generated by the thermal energy generated by the
electrothermal transducer element is brought into communication
with ambience while an internal pressure of the bubble is less than
an ambient pressure, and wherein droplets having volumes not more
than 15.times.10.sup.-15 m.sup.3 are ejected at a frequency not
less than 7 kHz, said method comprising:
a first step, wherein liquid remaining in the ejection outlet after
fluid communication with the ambience of the bubble maintains fluid
communication with liquid retracted from the ejection outlet in the
liquid flow path;
a second step, wherein the liquid remaining in the ejection outlet
and the liquid retracted from the ejection outlet in the liquid
flow path are merged to refill the liquid into the ejection outlet;
and
a third step of repeating said first and second steps to eject
droplets having volumes not more than 15.times.10.sup.-15 m.sup.3
at a frequency not less than 7 kHz.
26. A method according to claim 25, wherein a sum of the distance
between an upper surface and a lower surface of the ejection outlet
and a height of the liquid flow path is not more than a size of the
electrothermal transducer element.
27. A method according to claim 25, wherein an outer surface in
which the ejection outlets are formed is treated to be
hydrophobic.
28. A method according to claim 27, wherein an outer surface in
which the ejection outlets are formed has a partial hydrophobic
region.
29. A method according to claim 25, wherein an inner surface in
which the ejection outlets are formed is treated to be hydrophilic.
Description
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to a liquid ejecting method and a
liquid ejecting head which are used for ejecting droplets of liquid
such as ink toward various recording media, such as paper, for the
purpose of recording. In particular, it relates to a liquid
ejecting method for ejecting extremely small droplets of liquid at
an extremely high frequency, and also, a liquid ejecting head, that
is, a recording head, which comprises a plurality of liquid paths
arranged at a high density to realize high resolution.
Among various liquid ejecting methods are so-called bubble jet type
liquid ejecting methods. According to these methods, bubbles are
rapidly grown in liquid, and the pressure generated by the bubble
grown is used to eject droplets of liquid from liquid ejection
orifices. These methods are high in liquid ejection response, and
therefore, are excellent for high speed recording and high density
recording.
Among the bubble jet type liquid ejecting methods are liquid
ejection methods which allow a bubble generated on a heat
generating member to open to the atmosphere at the edge of an
ejection orifice. As for such methods, Japanese Laid-Open Patent
Application No 10940/1992, 10941/1992, 10742/1992, and the like,
are well known.
These methods have following characteristics. First, they can
increase liquid ejection velocity, and therefore, can increase
reliability. Secondly, they can eject substantially the entire
liquid present between a heat generating member and an ejection
orifice, and therefore, can unify the volume by which liquid is
ejected each time, which in turn reduces irregularity in terms of
the image density.
As recording technologies progress, it has come to be required to
record extremely high quality images, that is, to deposit liquid
droplets of an extremely small volume (for example,
1.5.times.10.sup.-10 m.sup.3 or less) on recording medium at an
extremely high density (for example, 600 dots/25.4 mm or more). In
order to record such highly precise images, ejection orifices, and
liquid paths leading to the ejection orifices, must be arranged at
an extremely high density. For example, in order to accomplish the
aforementioned recording density of 600 dots/25.4 mm, the ejection
orifices must be aligned in two parallel lines, at a density of 300
unit/25.4 mm, the units in one line being displaced by half a pitch
from the units in the other line in the line direction.
Recording an image with the use of finer liquid droplets increases
the number of liquid droplets to be ejected, which in turn reduces
recording speed. In order to prevent this recording speed
reduction, it is necessary to increase the frequency at which
liquid droplets are ejected from each ejection orifice per unit of
time (hereinafter, "ejection frequency"). For example, in the case
of the structure described above, the ejection frequency must be at
least 7 kHz.
Further, in order to record a high quality image by ejecting liquid
droplets with a volume as small as the one described above, the
reliability with which liquid droplets are ejected must be
improved.
As described above, there are bubble jet type liquid ejecting
method which allow bubbles to become connected to the atmosphere.
For example, Japanese Lair-Open Patent Application No. 16365/1993
discloses a technology regarding the state of a liquid droplet at
the time of ejection, and the condition for allowing a bubble to
become connected to the atmosphere.
When a bubble jet type liquid ejecting method which allows a bubble
to become connected to the atmosphere was applied to an ink jet
head which ejected extremely small liquid droplets with a volume of
1.5.times.10.sup.-10 m.sup.3, it was confirmed that during a
recording operation, liquid droplets suddenly failed to be ejected
from some of the ejection orifices through which liquid droplets
had been properly ejected. This phenomenon was different from the
ejection failure which occurred to the prior liquid ejecting heads.
The investigation of this phenomenon revealed the following. That
is, recording liquid suddenly plugged the ejection orifices during
the period between the time when a bubble became connected to the
atmosphere and the time when the refilling ended. Thereafter,
recording liquid could not be ejected from the plugged ejection
orifices unless a recovery operation was carried out with the use
of the recovery mechanism of the main assembly of an image forming
apparatus.
FIG. 5 is a section of a liquid ejection orifice, and a liquid path
leading to the orifice, which depicts the above described
phenomenon. As is evident from FIG. 5, immediately after a bubble
becomes connected to the atmosphere and a droplet of recording
liquid 501 is ejected, an ejection orifice is plugged with
recording liquid 501. At this point of time, there also remains
recording liquid 501 in the ink supply path. However, there is no
recording liquid adjacent to an electrothermal transducer 1,
because it is immediately after liquid ejection. In other words,
there is only atmospheric air 502 adjacent to the electrothermal
transducer 1. In this state, even if an electrical pulse is applied
to the electrothermal transducer 1, a droplet of recording liquid
501 cannot be ejected, since there is no recording liquid 501
around the electrothermal transducer 1. Therefore, it is impossible
to unplug the ejection orifice 4.
Further, during the development of the present invention, it became
evident that when the aforementioned type of head, in which a large
number of liquid paths were disposed at a high density, was driven
at a high frequency, attention must be paid to the state of the
meniscus after a bubble became connected to the atmosphere, in
particular, how the state of the meniscus after the connection is
different from the state of the meniscus prior to the connection.
Thus, the object of the present invention is to provide a reliable
liquid ejection method, that is, a liquid ejecting method which
does not suddenly fail to eject liquid, i.e., a liquid ejecting
method which makes high speed recording possible with the use of a
bubble jet type liquid ejecting head, in particular, so-called side
shooter type liquid ejecting head in which ejection orifices for
ejecting extremely small liquid droplets at a high frequency are
disposed at a high density, directly facing heat generating members
one for one, and in which a bubble is allowed to become connected
to the atmosphere.
SUMMARY OF THE INVENTION
The gist of the present invention for accomplishing the
above-described object of the present invention is as follows.
The liquid ejecting method in accordance with the present invention
uses a liquid ejecting head which comprises a plurality of
electrothermal transducers capable of generating a sufficient
amount of thermal energy for generating bubbles in liquid, a
plurality of ejection orifices disposed directly facing the
electrothermal transducers one for one, and a plurality of liquid
paths. The ejection orifices are aligned at a density of no less
than 300 per 25.4 mm, and are connected to the liquid paths one for
one. This liquid ejecting method is characterized in that bubbles
generated by the thermal energy generated by an electrothermal
transducer eject droplets of liquid with a volume of no more than
15.times.10.sup.-15 m.sup.3, one for one, at a frequency of no less
than 7 kHz, and open to the atmosphere as they eject the liquid
while their internal pressure is below the atmospheric pressure,
and that the height of the liquid path in the liquid ejecting head
is no less than 6 .mu.m, and the distance between the top and
bottom openings of the ejection orifice is no more than half the
minimum distance across the ejection orifice through the center of
the orifice.
The liquid ejecting head in accordance with the present invention
comprises a plurality of electrothermal transducers capable of
generating thermal energy for generating bubbles in liquid, a
plurality of ejection orifices disposed directly facing the
electrothermal transducers one for one, and a plurality of liquid
paths. The ejection orifices and liquid paths are aligned at a
density of no less than 300 per 25.4 mm. To the electrothermal
transducers, driving signals are applied at a frequency of no less
than 7 kHz. This liquid ejecting head is characterized in that
bubbles are generated in the liquid paths, and eject droplets of
liquid with a volume of no more than 15.times.10.sup.-15 m.sup.3,
one for one, opening to the atmosphere as they eject the liquid
while their internal pressure is below the atmospheric pressure,
and that the height of the liquid path is no less than 6 .mu.m, and
the distance between the top and bottom openings of the ejection
orifice is no more than half the minimum distance across the
ejection orifice through the center of the orifice.
Further, the liquid ejecting method in accordance with the present
invention uses a liquid ejecting head which comprises a plurality
of electrothermal transducers capable of generating a sufficient
amount of thermal energy for generating bubbles in liquid, a
plurality of ejection orifices disposed directly facing the
electrothermal transducers one for one, and a plurality of liquid
paths. The ejection orifices are aligned at a density of no less
than 300 per 25.4 mm, and are connected to the liquid paths one for
one. The bubbles generated by the thermal energy generated by an
electrothermal transducer eject droplets of liquid with a volume of
no more than 15.times.10.sup.-15 m.sup.3, one for one, at a
frequency of no less than 7 kHz, and open to the atmosphere as they
eject the liquid while their internal pressure is below the
atmospheric pressure. This liquid ejecting method is characterized
in that it comprises a process in which the liquid which remains
within the ejection orifice after the bubble opens to the
atmosphere, remains in connection to the liquid in the liquid path
which retracts away from the ejection orifice, and a process in
which the liquid remaining in the ejection orifice joins with the
liquid in the liquid path, and refills the ejection orifice.
With the provision of the above described structure, the ejection
orifices in a side shooter type liquid ejecting head in which
bubbles open to the atmosphere are not plugged with recording
liquid. Consequently, the appearance of the unwanted white lines
during recording, for which the sudden ejection failure of some of
the ejection orifices is responsible, is reliably prevented, making
it possible to reliably print high quality images at a high
speed.
These and other objects, features and advantages of the present
invention will become more apparent upon a consideration of the
following description of the preferred embodiments of the present
invention taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1, (a) is an external perspective view of a liquid ejecting
head to which the liquid ejecting method in accordance with the
present invention can be applied, and depicts the general structure
of the head. FIG. 1, (b) is a section of the liquid ejecting head
in FIG. 1, (a), at a line A--A, and depicts the general structure
of the head.
FIG. 2, (a) is a vertical section of the essential portions, that
is, one of the ejection orifices and one of the liquid paths, of
the liquid ejecting head in FIG. 1. FIG. 2, (b) is a top view of
the essential portion of the liquid ejecting head illustrated in
FIG. 2, (a).
FIG. 3, (a)-(g), are sections of the essential portions of the
liquid ejecting head to which the liquid ejecting method in
accordance with the present invention is applicable, and depict the
operational steps of the head.
FIG. 4 is a partially broken perspective view of an example of a
liquid ejecting apparatus compatible with a liquid ejecting head to
which the liquid ejecting method in accordance with the present
invention is applicable, and depicts the general structure
thereof.
FIG. 5 is an enlarged section of the essential portion of a liquid
ejection head in accordance with the present invention, and depicts
the problem which is solved by the present invention.
FIG. 6 is a section of a liquid ejecting recording head in
accordance with the present invention, and depicts the vertically
tapered shape of the ejection orifice.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiment 1
FIG. 1, (a) is an external perspective view of a liquid ejecting
head to which the liquid ejecting method in accordance with the
present invention can be applied, and depicts the general structure
of the head. FIG. 1, (b) is a section of the liquid ejecting head
in FIG. 1, (a), at a line A--A, and depicts the general structure
of the head. In FIG. 1, a referential code 2 designates a substrate
formed of Si, on which electrothermal elements as heaters, and
ejection orifices, have been formed by a thin film technology. The
electrothermal elements and ejection orifices will be described
later in detail. On this element substrate 2, a plurality of
ejection orifices are aligned in two parallel lines, so that the
ejection orifices 4 in one line are displaced by half a pitch from
the ejection orifices 4 in the other lines, in the line direction,
like footprints of a bird, as shown in FIG. 1, (a). The element
substrate 2 is fixed to a portion of an L-shaped supporting member
102 with glue. Also fixed to the supporting member 102 is a wiring
substrate 104, the wiring on which is electrically connected to the
wiring on the element substrate 2 by bonding. The supporting member
104 is formed of aluminum in view of processability. A referential
character 103 designates a molded member, into which the supporting
member 102 is partially inserted to be supported by the molded
member 103. The molded member 103 comprises a liquid supply path
107, through which liquid (for example, ink) is supplied from a
liquid storing portion (unillustrated) to the ejection orifices
with which the aforementioned element substrate 2 is provided.
Further, the molded member 103 functions as a member which plays a
role in removably installing the entirety of a liquid ejecting head
in accordance with the present invention into a liquid ejecting
apparatus, and removably fixing it to the liquid ejecting
apparatus. The liquid ejecting apparatus will be described later in
detail.
The element substrate 2 comprises a connective path 105, which
penetrates through the element substrate 2, and through which the
liquid supplied through the liquid supply path 107 of the molded
member 103 is supplied to the ejection orifices. The connective
path 105 is connected to liquid paths leading to ejection orifices,
one for one, and also functions as a common liquid chamber.
FIG. 2, (a) is a vertical section of the essential portions, that
is, the ejection orifice and the liquid path, of the liquid
ejecting head in FIG. 1. FIG. 2, (b) is a top view of the essential
portion of the liquid ejecting head illustrated in FIG. 2, (a).
As illustrated in FIG. 2, the liquid ejecting head in accordance
with the present invention is provided with rectangular
electrothermal elements as heaters 1, which are disposed at
predetermined locations, one for one, on the element substrate 2.
Above the heaters 1, an orifice plate 3 is disposed. The orifice
plate 3 is provided with rectangular ejection orifices 4, which
directly face the center portions of the heaters 1, one for one.
The size of the opening of the ejection orifice 4 is designated by
a referential code So as can be seen in FIG. 2, (b). Referential
characters 41 and 42 designate the top and bottom "surfaces" of the
ejection orifice 4. In this embodiment, the top and bottom
"surfaces" are imaginary surfaces: the imaginary surfaces formed by
extending the top and bottom surfaces of the orifice across the top
and bottom openings of the ejection orifice 4.
Referring to FIG. 2, (a), the gap between the heater 1 and the
orifice plate 3 equals the height Tn of the liquid path 5, and is
determined by the height of a liquid path wall 6. Referring to FIG.
2, (b), in which the liquid path 5 extends in the direction
indicated by an arrow mark X, the ejection orifices 4 which are in
connection to the liquid paths 5 one for one are aligned in a
plurality of parallel lines perpendicular to the direction X. The
plurality of liquid paths 5 are connected to the connective path
105, in FIG. 1, (b), which also functions as a common liquid
chamber. The thickness of the orifice path 3, which equals the
distance between the imaginary top and bottom surfaces 41 and 42 of
the ejection orifice, is designated by a referential character
To.
Next, an embodiment of the liquid ejecting method in accordance
with the present invention, which uses a liquid ejecting head with
the above described structure, will be described.
FIG. 3, (a)-(g), are sections of the essential portions of the
liquid ejecting head to which the liquid ejecting method in
accordance with the present invention is applicable. They depict
the operational steps of the head.
Referring to FIG. 3, (a), in the normal state, a meniscus 11 is at
the top end of the ejection orifice. In this state, driving voltage
is applied to the heater 1. The heater 1 is desired to be driven
with the use of short pulses so that the meniscus is prevented from
being excessively retracted by the excessive bubble growth. The
duration of the electrical pulse applied to the heater 1 to eject
liquid is desired to be no more than 3.5 .mu.sec. This is due to
the following reason. If the pulse duration is greater than 3.5
.mu.sec., bubble growth becomes excessive, which makes the location
of the meniscus after the liquid ejection excessively far from the
ejection orifice. As a result, refilling time becomes longer, which
makes the liquid ejecting head unsuitable for high speed recording.
It is possible to use a multi-pulse driving method, that is, a
driving method which applies two or more pulses per ejection. In
such a case, the duration of the pre-pulse, that is, the pulse
applied prior to the application of the main pulse for recording
liquid ejection, is desired to be no more than 1.5 .mu.sec. The
interval between the pre-pulse and the main pulse is desired to be
no more than 2.0 .mu.sec. If the duration of the pre-pulse exceeds
1.5 .mu.sec, and/or the interval between the pre-pulse and the main
pulse exceeds 2.0 .mu.m, bubble growth becomes excessive, which in
turn causes the meniscus to retract by a greater distance. The
greater retraction of the meniscus makes it impossible for the
liquid ejection head to eject liquid at a high frequency; in other
words, it makes the objects of the present invention impossible to
accomplish.
The proper driving voltage value for accomplishing the objects of
the present invention is 1.1 to 1.3 times the threshold voltage Vth
for liquid ejection. If the driving voltage is no more than 1.1
times the threshold voltage Vth, liquid ejection velocity is
excessively low, causing liquid droplets to be ejected off the
predetermined course, provided that bubbles are generated and
liquid droplets are ejected. Also, liquid ejection becomes instable
at a high frequency. On the contrary, if the driving voltage value
is no less than 1.3 times the threshold voltage Vth, bubble length
becomes excessive, causing the meniscus to retract by a greater
distance, which in turn prolongs refilling time, and/or excessively
increases liquid ejection velocity, increasing the amount of the
splash which occurs as a liquid droplet hits the recording medium.
Thus, the aforementioned driving voltage range is one of the
desirable conditions for the present invention.
Next, referring to FIG. 3, (b), as driving voltage is applied to
the heater 1, a bubble 301 is caused to grow in the liquid, in
contact with the heater 1. Then, as the bubble 301 grows, the
liquid in the ejection orifice 4 and liquid paths 5 on the top side
of the bubble 301 swells upward from the top end of the ejection
orifice 4. During this process, the pressure of the bubble 301,
which has been greater than the atmospheric pressure, begins to
drop below the atmospheric pressure.
Next, referring to FIG. 3, (c), as the bubble grows further, the
liquid in the ejection orifice 4 and liquid path 5 on the top side
of the bubble 301 is ejected upward from the top end of the
ejection orifice 4. At this stage, however, the bubble 301 has not
become connected to the atmosphere, and the meniscus in the liquid
path 5 keeps retracting due to the further growth of the bubble
301. Immediately before the bubble 301 becomes connected to the
atmosphere, the front end of the recording liquid, in terms of
liquid flow, in the liquid path 5 is still at the imaginary bottom
surface 42 of the ejection orifice 4, and also is in connection
with the recording liquid remaining on the internal surface of the
ejection orifice 4. The internal pressure of the bubble 301 remains
below the atmospheric pressure until the bubble 301 becomes
connected to the atmosphere. If the bubble 301 becomes connected to
the atmosphere while the internal pressure of the bubble 301 is
equal to, or above, the atmospheric pressure, the instable liquid
adjacent to the ejection orifice 4 is caused to splash at the time
of the connection between the bubble 301 and the atmosphere.
Further, there is no force which works to pull the instable liquid
back into the liquid path, and therefore, the instable liquid
adjacent to the ejection orifice 4 cannot be prevented from
splashing.
Next, referring to FIG. 3, (d), at the same time or immediately
after the bubble 301 becomes connected to the atmosphere, the
liquid droplet 12 is ejected from the ejection orifice 4, and
leaves the top edge of the ejection orifice 4. At this moment when
the liquid droplet leaves the top edge of the ejection orifice 4,
if the separation of the liquid droplet from the liquid in the
ejection orifice 4 occurs on the left-hand side, in the drawing, of
the ejection orifice 4, the major portion of the aforementioned
recording liquid which remains on the internal surface of the
ejection orifice is pulled down by the recording liquid which is
remaining at the aforementioned imaginary bottom surface 42 of the
ejection orifice, and eventually joins with the recording liquid
within the liquid path 5, that is, returns to the liquid path 5.
The meniscus 11 retracts farthest slightly after the connection
between the bubble 301 and the atmosphere. After this point, the
liquid droplet is ejected as shown in FIG. 3, (e)-(g). Then, the
recording liquid refills the ejection orifice 4, and
stabilizes.
Regarding the above described processes, as long as the liquid
which retains within the ejection orifice after the connection
between the bubble and the atmosphere remains in connection with
the liquid which retracts into the liquid path from the ejection
orifice, and this liquid remaining within the ejection orifice is
caused to join with the liquid within the liquid path and
eventually refill the ejection orifice, even if recording liquid is
adhering adjacent to the imaginary top surface of the ejection
orifice, this liquid joins with the aforementioned recording liquid
which is adhering to the internal surface of the ejection orifice.
In other words, even this liquid, which is adhering adjacent to the
imaginary top surface of the ejection orifice, is moved back into
the liquid path 5, the aforementioned phenomenon that the recording
liquid fails to be ejected does not occur, that is, the recording
liquid is reliably ejected.
The volume by which liquid is ejected as the liquid droplet 12 is
determined by the ejection orifice size or the like of a liquid
ejecting head used for liquid ejection. In the case of the liquid
ejecting head in this embodiment, the volume of the liquid droplet
12 is made to be no more than 15.times.10.sup.-15 m.sup.3.
One of the desirable conditions for allowing the bubble 301 to
reliably become connected to the atmosphere is: To+Tn.ltoreq.heater
size. The heater size means Sh.sup.1/12, in which Sh stands for the
size of the heating surface of the heater.
If To+Tn.gtoreq.Sh.sup.1/2, the greater the value of (To+Tn) in
relative terms, the more likely are the factors responsible for the
connection between the bubble and the atmosphere to negatively work
in terms of the balance in the connection, even if the bubble
becomes connected to the atmosphere. Therefore, a proper relation
between (To+Tn) and Sh.sup.1/2 becomes the desirable condition. In
addition, if To+Tn.gtoreq.Sh.sup.1/2, the recording liquid is
ejected without allowing the bubble to become connected to the
atmosphere. In other words, one of the prerequisites of the present
invention does not exist.
The structural features of a liquid ejecting head described below
are listed as the embodiments of the present invention which assure
that the aforementioned liquid, which remains within a nozzle after
the connection between a bubble and the atmosphere, remains in
connection with the aforementioned liquid which retracts from the
ejection orifice into the liquid path, and that this liquid
remaining in contact with the liquid having retracted from the
ejection orifice joints with the liquid within the liquid path, and
refills the nozzle.
(1) An ejection orifice is more effective if its vertical section
is tapered, that is, the minimum distance across the aforementioned
imaginary top surface of the ejection orifice through the center of
the top surface is shorter than the minimum distance across the
aforementioned imaginary bottom surface of the ejection orifice
through the center of the bottom surface.
FIG. 6 depicts the vertical section of an ejection orifice with the
above described vertical section. Since the ejection orifice is
tapered, it is geometrically easier for the recording liquid
remaining on the internal surface of the ejection orifice to remain
in connection with the ink within the liquid path. Further, since
the size of the imaginary bottom surface of the ejection orifice is
greater than the imaginary top surface of the ejection orifice, it
is more difficult for the recording liquid to plug the ejection
orifice.
(2) An ejection orifice is more effective if its horizontal section
is in the form of a star rather than in the form of a circle or a
square. In other words, the easier it is for the recording liquid
to remain on the internal surface of the ejection orifice, the
easier it is for the aforementioned processes to occur. This is due
to the following reason. That is, if the horizontal section of an
ejection orifice is in the form of a "star", it is easier for
recording liquid, which is adhering to the top portion of the
ejection orifice, to remain in connection to the liquid at the
bottom portion of the ejection orifice, and also, the effective
horizontal size of the ejection orifice at the time of liquid
ejection is determined by the size of the area surrounded by the
lines which connect the adjacent inward corners of the star.
(3) Meniscus curvature
In order to make it easier for the liquid remaining on the internal
surface of an ejection orifice to join with the liquid within the
liquid path, the negative pressure which the meniscus formed in the
liquid path generates is desired to be as large as possible. In
order for the meniscus to generate a greater amount of negative
pressure, it is desired that the liquid path is as small as
possible in height and cross section, as long as refilling time
does not excessively increase.
In order to enhance the desirable embodiments (1)-(3), it is
particularly desirable that the bottom side of an ejection orifice
is rendered easier to be wetted by recording liquid (orifice is
treated so that it becomes hydrophilic).
(4) Volume of liquid adhering to ejection orifice
In order to prevent recording liquid from plugging an ejection
orifice, it is desirable that control is executed so that the
amount of liquid which remains at the top portion of the ejection
orifice, that is, the amount of excessive liquid, becomes as small
as possible. In order to do so, it is important that the top
portion of the ejection orifice is treated to give it
hydrophobicity, so that small patches of liquid which are adhering
to the top portion of the internal surface of the ejection orifice
are prevented from joining with each other and growing. In other
words, it is important that the top portion of the internal surface
of the ejection orifice is rendered as water repellent as possible.
Further, it is effective for hydrophilic regions to be located away
from the water repellent top portion of the ejection orifice.
Next, the factors which determine how liquid is ejected from the
above described liquid ejecting head will be described in more
detail from the standpoint of orifice plate configuration.
The liquid which remains on the internal wall of an ejection
orifice after recording liquid ejection forms a meniscus within the
ejection orifice. At this moment, the relative pressure P of the
recording liquid is:
in which r1 stands for 1/2 of the minimum distance across the
meniscus formed in the ejection orifice, through the center of the
meniscus, as seen from above; r2 stands for the radius
corresponding to the curvature of the meniscus (curvature of the
section of the meniscus, at a plane which is parallel to a liquid
path, and contains the center of the meniscus); and .gamma. stands
for the surface tension of the recording liquid. The ejection
orifice is less likely to be plugged with the recording liquid when
P>0, because even if recording liquid is present adjacent to the
imaginary top surface of the ejection orifice, this liquid is more
difficult to pull into the ejection orifice when P>0.
The value of r1 is proportional to ejection orifice diameter
(.apprxeq.So.sup.1/2), and the value of r2 is proportional to the
thickness To.
Based on the above described relation, the inventors of the present
invention tested various liquid ejecting heads produced in
consideration of the structural requirements which prevents the
aforementioned phenomenon that an ejection orifice is plugged with
recording liquid, that is, the requirement regarding the minimum
distance across the horizontal cross section of the ejection
orifice through the center of the cross section, and the orifice
plate thickness. As a result, it was discovered that even if the
liquid within the liquid path is not in contact with the imaginary
bottom surface of the ejection orifice immediately after the
connection between a bubble and the atmosphere, the ratio at which
the aforementioned phenomenon, or the plugging of the ejection
orifice by the recording liquid occurs, drastically differs across
a point at which the minimum distance across the horizontal section
of the ejection orifice through the center of the section is twice
the orifice plate thickness. That is, when the minimum distance
across the horizontal section of the ejection orifice through the
center of the section is twice the orifice plate thickness or
greater (if the distance between the imaginary top and bottom
surfaces of the ejection orifice is no more than half the minimum
distance across the horizontal section of the ejection orifice
through the center of the section), the ratio at which the
aforementioned phenomenon occurs is extremely low. On the contrary,
if the minimum distance across the horizontal section of the
ejection orifice through the center of the section is no more than
the twice the orifice plate thickness (if the distance between the
imaginary top and bottom surfaces of the ejection orifice is no
less than half the minimum distance across the horizontal section
of the ejection orifice through the center of the section), the
ratio at which the aforementioned phenomenon occurs is extremely
high, that is, high enough to create problems in terms of practical
usage.
In the present invention, if the aforementioned horizontal section
of an ejection orifice, which is perpendicular to the direction in
which recording liquid is ejected, is substantially in the form of
a true circle, "the minimum distance across the horizontal section
of the ejection orifice through the center of the section" can be
defined as the diameter of the virtually circular horizontal
section of the ejection orifice. If the horizontal section of the
ejection orifice is square, it can be defined as the length of one
of the four sides; if rectangular, it can be defined as the length
of the shorter side; if oval, it can be defined as the length of
its shortest diameter; and if the vertical section of an ejection
orifice, parallel to the ejecting direction, has a tapered shape,
it can be defined as the minimum distance across the ejection
orifice through the center of the ejection orifice.
Next, the conditions required to drive a liquid ejecting head at a
high frequency will be described. In order to drive a liquid
ejecting head at a high frequency, refilling time must be short.
Refilling time is determined by (1) the maximum amount of meniscus
retraction, (2) capillary force as the force for driving the liquid
for refilling, and (3) viscous resistance of the liquid path during
refilling.
The smaller the maximum amount of meniscus retraction (2), the
shorter the refilling time. Thus, the amount of meniscus retraction
is desired to be as small as possible as long as liquid droplets
with a desirable volume are reliably ejected. In order to satisfy
this requirement, it is desirable that the duration of a driving
pulse is set to be no more than 3.5 .mu.sec.
The capillary force (2) is the force which drives ink during
refilling, and therefore, generally speaking, it is desired to be
as large as possible. In other words, the surface tension of
recording liquid is desired to be as high as possible, preferably,
no less than 0.025 N/m.
Generally speaking, the viscous resistance of a liquid path (3) is
desired to be as small as possible.
The above described conditions are for the purpose of making it
easier for the aforementioned liquid within the liquid path to
remain in connection to the liquid which remains on the internal
surface of an ejection orifice. Therefore, when these conditions
are satisfied, a liquid ejecting head in accordance with the
present invention can more reliably eject recording liquid.
It should be noted here that the capillary force (2) and the
viscous resistance (3) of a liquid path must be set so that the
meniscus vibration does not become excessively large after the
completion of refilling.
Regarding a condition which reduces the viscous resistance of the
liquid path during refilling to a practical level at which a liquid
ejecting head in accordance with the present invention can be
driven at a high frequency, it is discovered that the height Tn of
the liquid path must be 6 .mu.m or more; 6 .mu.m.ltoreq.Tn. If 6
.mu.m.gtoreq.Tn, that is, if the height of the liquid path is
excessively reduced, the viscous resistance of the liquid path
excessively increases, prolonging refilling time, and therefore,
the liquid ejecting head cannot be driven at high frequency. In
order to keep the viscous resistance of the liquid path low, it is
necessary to employ recording liquid, the viscosity of which is not
excessively high. In other words, the viscosity of the recording
liquid is desired to be no more than 5.times.10.sup.-2 N/s.
In order to print an image desirable in terms of the distortion,
that is, an image which is small in the amount of distortion, the
velocity at which liquid droplets are ejected is desired to be no
less than 10 m/sec and no more than 30 m/sec, preferably, no less
than 10 m/sec and no more than 20 m/sec. If the velocity at which
liquid droplets are ejected is less than 10 m/sec, liquid droplets
are likely to miss the intended spots on the recording medium,
which is possible to reduce print quality. If the ejection velocity
exceeds 30 m/sec, the ejected liquid droplets are likely to splash
and form mist as they hit the recording medium. Further, even if
the above described condition regarding the liquid ejection
velocity is satisfied, if the thickness of the orifice plate is
excessively reduced, it is possible that the direction in which
liquid droplets are ejected becomes instable, and also that the
mechanical strength of the orifice plate 3 is reduced. Thus, the
orifice plate needs to have a certain amount of thickness. More
specifically, the thickness of the orifice plate needs to be no
less than 4 .mu.m.
A liquid ejecting head in accordance with the above described
embodiments of the present invention, can be mounted in a liquid
ejecting apparatus, for example, the one illustrated in FIG. 4, to
practice the liquid ejecting method in accordance with the present
invention.
Next, an example of a liquid ejecting apparatus will be described
with reference to FIG. 4.
Referring to FIG. 4, a referential character 200 designates a
carriage on which the aforementioned liquid ejecting head is
removably mounted. In this liquid ejecting apparatus, four liquid
ejecting heads are employed to accommodate inks of different
colors, and are mounted on the carriage 200, along with an ink
container 201Y for yellow ink, an ink container 202M for magenta
ink, an ink container 201C for cyan ink, and an ink container 201B
for black ink.
The carriage 200 is supported by a guide shaft 202, and is enabled
to shuttle along the guide shaft 202, by an endless belt 204 driven
forward or backward by a motor 203. The endless belt is wrapped
around pulleys 205 and 206.
A sheet of recording paper P as recording medium is intermittently
conveyed in the direction indicated by an arrow mark B, which is
perpendicular to the direction A. The recording paper P is held by
being pinched by the upper pair of rollers 207 and 208, and the
bottom pair of rollers 209 and 210, being thereby given a certain
amount of tension so that it remains flat while being conveyed. The
roller units are driven by a driving section 211. However, the
apparatus may be structured so that the roller units are driven by
the aforementioned motor.
The carriage 200 stops at the home position at the beginning of
each printing operation, and also as necessary. At the home
position, capping members 212 for capping the four heads one for
one are located. The capping members 212 are connected to vacuuming
means, which prevents ejection orifices from being clogged, by
vacuuming the ejection orifices.
(Embodiments 1 and 2)
The liquid ejecting head illustrated in FIG. 2, (a) and (b), was
produced, and its performance was tested. The results are given in
Table 1. The ejection orifices were aligned in two parallel lines,
the ejection orifices in one line being displaced in the line
direction half a pitch from the ejection orifices in other line, as
shown in FIG. 1, (a) and (b). More specifically, in each line, the
ejection orifices are disposed at a pitch of 300 dpi, and the
ejection orifices in one line are displaced by 25.4 mm in line
direction, from ejection orifices in the other line. In other
words, the ejection orifices are arranged like the footprints of a
bird. Consequently, the ejection orifice density in the direction
perpendicular to the primary scanning direction of the head became
600 dpi (600 ejection orifices per 25.4 mm). The minimum distance
across the horizontal section of the ejection orifice through the
center of the section was 22 .mu.m, and the ejection orifices were
shaped so that their horizontal sections became square. The size So
of the opening of each ejection orifice was 484 .mu.m.sup.2 (=22
.mu.m.times.22 .mu.m). With this specification, the length of the
effective bubble generating region in the liquid flow direction was
26 .mu.m, and the distance from the center of the effective bubble
generating region to the edge of the effective bubble generating
region, on the liquid supply source side, was 13 .mu.m. The size of
the heating surface of each heater was 936 .mu.m.sup.2 (=26
.mu.m.times.36 .mu.m).
In Embodiments 1 and 2, the height Tn of the liquid flow path was
made to be 12 .mu.m and 6 .mu.m, respectively, and the thickness To
of the orifice plate was made to be 9 .mu.m and 11 .mu.m,
respectively. Further, across the surface of each heater, a 0.6
.mu.m thick electrically insulative film (SiO.sub.2) and a 0.3
.mu.m thick passivation film (Ta) were formed.
As for recording ink, the ink with the following composition was
used:
TiO glycol 15% Glycerin 5% Urine 5% Isopropyl alcohol 4% Water
remainder
The ink had a viscosity of 1.8.times.10.sup.-2, a surface tension
of 0.038 N/m, and a density of 1040 kg/m.sup.3.
The liquid ejecting head (recording head) structured as described
above was driven at 7 kHz with the use of a power source which
could apply a voltage Vop of 12 V to the heater. The duration of
the driving pulse was set to be 1.9 .mu.sec. When the duration of
the driving pulse applied to the heater was 1.9 .mu.sec, the
minimum voltage Vth (threshold voltage) necessary for the ink to be
ejected was 9.9 V. Therefore, Vop/Vth was 1.21. The performance, or
characteristic, regarding various aspects of this head, which was
realized when the head was driven under the above described
condition, is given in Table 1.
TABLE 1 Emb. 1 Emb. 2 Comp. 1 Comp. 2 Comp. 3 Comp. 4 Sh
(.mu.m.sup.2) 936 ditto ditto ditto ditto ditto So (.mu.m.sup.2)
484 ditto ditto ditto 441 484 (22 .mu.m .times. 22 .mu.m) (21 .mu.m
.times. 21 .mu.m) (22 .mu.m .times. 22 .mu.m) D (.mu.m) 22 ditto
ditto ditto 21 22 Tn (.mu.m) 12 6 6 4 6 5.5 To (.mu.m) 9 11 12 9 11
11 Stability good good no good no good no good no good Vol.
(10.sup.-18 m.sup.3) 8.2 7.7 7.5 7.2 7.4 7.9 Speed (m/s) 15.8 17.3
18.0 19.1 17.7 17.8 Refilling time (.mu.sec) 75 129 146 280 127 159
Properly continued to 170 6 0.2 0.4 0.7 0.2 D/To 2.44 2.0 1.83 2.44
1.91 2.0
Under the above described conditions, a printing operation was
carried out, in which a plurality of A3 size sheets or recording
paper were continuously fed. The minimum cross distance D of an
ejection orifice through the center of the orifice was 22 .mu.m,
which was no less than twice the orifice plate thickness To which
equaled the distance between the imaginary top and bottom surfaces
of the ejection orifice. The performance was such that printing
could be carried out across the entire surface of an A3 sheet of
recording paper or more, without an interruption, which exceeded a
performance level above which there would be no problem in
practical usage. In other words, the head was reliable.
The head was fast enough in ink ejection velocity to deal with a
situation in which ink viscosity had increased while the head was
left unused. More specifically, the head could desirably deal with
ink, the viscosity of which was as high as 5.times.10.sup.-1 poise.
When the ink viscosity increased beyond 10.times.10.sup.-1 poise,
that is, when the ink viscosity was excessively high, ink ejection
velocity dropped below 10 m/sec. As a result, ink droplets missed
intended spots on the recording medium. In order to assure that the
object of the present invention is accomplished, the surface
tension of ink is desired to be as high as possible. However, the
surface tension of the ink must be determined in consideration of
how an ink droplet behaves as it hits the recording medium, in
addition to the ink ejection velocity. Thus, the surface tension of
ink is desired to be no less than 30.times.10.sup.-2 N/m, and there
is no restriction regarding the upper limit as long as the ink can
be desirably ejected by a bubble. If the surface tension of the ink
is less than 30.times.10.sup.-2 N/m, the capillary force generated
by the ink is not high enough to serve as the force for driving the
ink for refilling. Therefore, refilling time is long, and long
refilling time makes it impossible for the head to be driven at a
high frequency, which is a problem.
In the above embodiments, the refilling time was 75 .mu.sec,
counting from the beginning of the liquid ejection pulse
application. The meniscus vibration thereafter was at an
undetectable level, and had virtually no effect upon printing
quality.
Also in those embodiments, the heater protection film was rendered
thin, and the pulse duration was set short. Consequently, the
amount of bubble growth was relatively small. In other words, the
refilling time was reduced by reducing the amount of meniscus
retraction, instead of increasing refilling speed.
Further, the protective layer for the heater 1 was formed of
SiO.sub.2 (0.6 .mu.m thick), and passivation film (0.3 .mu.m thick)
was formed of Ta. These films are desired to be as thin as
possible, provided that heater durability is reasonably long.
Reducing the thickness of the protective layer makes it possible to
reduce the overall amount of the thermal energy conducted from a
heater to the ink between the beginning of the pulse application
and the beginning of bubble growth. Therefore, reducing the
thickness of the protective layer reduces the amount of bubble
growth after bubble generation, reducing consequently the amount of
meniscus retraction. When the protective layer is formed of
SiO.sub.2 or SiN, its thickness is desired to be no more than 1
.mu.m. Obviously, if extremely non-corrosive platinum or the like
material is used as heater material, the protective layer may be
eliminated.
In the liquid ejecting heads in accordance with the present
invention, in which a bubble generated in a liquid path becomes
connected to the atmosphere through an ejection orifice, the volume
by which ink is ejected per ejection is generally determined by the
geometric aspects of the heater, liquid path, and ejection orifice.
In other words, there is a wide range in the amount of bubble
growth, in which the volume by which ink is ejected per ejection is
not affected by the reduction in bubble growth.
(Comparative Examples 1-4)
The liquid ejecting heads employed in Comparative Examples 1-4 are
the same as those employed in Embodiments 1 and 2, except that in
these comparative examples, the height of the liquid path was
varied from the those in Embodiments 1 and 2. In other words, in
Embodiments 1 and 2, the height Tn of the liquid path was 12 .mu.m
and 6 .mu.m, whereas in Comparative Examples 1-4, it was 6 .mu.m, 4
.mu.m, 6 .mu.m and 5.5 .mu.m, correspondingly. In Comparative
Examples 1-4, the thickness To of the orifice plate was 12 .mu.m, 9
.mu.m, 11 .mu.m, and 11 .mu.m, correspondingly, and the minimum
distance across the opening of each ejection orifice through the
center of the orifice was less than twice the orifice plate
thickness To.
In Comparative Examples 1 and 3, in which the orifice plate
thickness To, which was set to be equal to the distance between the
imaginary top and bottom surfaces of each ejection orifice, was
greater than half the minimum distance D across the opening of the
ejection orifice through the center of the opening, the liquid
ejecting head frequency failed to eject the liquid, or the ink. In
Comparative Examples 2 and 4, in which the height of the liquid
path was less than 6 .mu.m, refilling time was so long that the
liquid ejecting head was not suitable for high frequency
driving.
Although this is not recorded in Table 1, if the value of (To+Tn)
is greater than Sh.sup.1/2 (.apprxeq.31 .mu.m), the behaviors of a
liquid droplet and a meniscus become instable at the time when the
bubble becomes connected to the atmosphere, which negatively
affects print quality.
(Embodiments 3-5 and Comparative Examples 5-10)
The liquid ejecting head illustrated in FIG. 2, (a) and (b), was
produced, and its performance was tested. The results are given in
Table 2. The ejection orifices were aligned in two parallel lines
as shown in FIG. 1, (a) and (b). More specifically, in each line,
the ejection orifices were disposed at a pitch of 600 dpi, and the
ejection orifices in one line were displaced by half a pitch, in
line direction, from ejection orifices in the other line. In other
words, the ejection orifices were arranged like the footprints of a
bird. Consequently, the ejection orifice density in the direction
perpendicular to the primary scanning direction of the head became
1200 dpi. The size of the opening of each ejection orifice in
Embodiments 3-5 was 227 .mu.m.sup.2, (=.phi.17 .mu.m), 225
.mu.m.sup.2 (=15 .mu.m square), and 234 .mu.m.sup.2. In each of
Embodiments 3-5, the size Sh of the heating surface of each heater
was 576 .mu.m.sup.2 (24 .mu.m.times.24 .mu.m).
In Embodiments 3-5, the same ink as the one employed in Embodiments
1 and 2 was employed.
As for the height Tn of each liquid path, it was made to be 12
.mu.m in Embodiments 3 and 4, and 6 .mu.m in Embodiment 5 . As for
the thickness To of the orifice plate, it was made to be 7 .mu.m in
Embodiment 3, and 6 .mu.m in Embodiment 4. In Embodiment 5, it was
made to be 9 .mu.m.
In Comparative Examples 5-10, the size So of each ejection orifice
was made to be 220 .mu.m.sup.2, 314 .mu.m.sup.2, 227 .mu.m.sup.2,
202 .mu.m.sup.2 (14.2 .mu.m square), 324 .mu.m.sup.2, and 324
.mu.m.sup.2, correspondingly. The size Sh of the heating surface of
each heater was made to be the same as that for Embodiments 3-5,
which was 570 .mu.m.sup.2 (24 .mu.m.times.24 .mu.m). The height Tn
of each liquid path in Comparative Examples 5-10 was made to be 12
.mu.m, 4 .mu.m, 8 .mu.m, 12 .mu.m, 6 .mu.m and 5.0 .mu.m,
correspondingly, and the thickness To of each orifice plate was
made to be 9 .mu.m, 11 .mu.m, 9 .mu.m, 9 .mu.m, and 9.5 .mu.m, and
9 .mu.m, correspondingly.
The sheet resistance of the heater was 53 ohm.
The liquid ejecting head (recording head) structured as described
above was driven at 10 kHz with the use of a power source which
could apply a voltage Vop of 9.0 V to the heater. The duration of
each driving pulse was set to be 2.7 .mu.sec. When the duration of
driving pulse applied to the heater was 2.7 .mu.sec, the minimum
voltage Vth (threshold voltage) necessary for the ink to be ejected
was 7.2 V. Therefore, Vop/Vth was 1.25. The performance, or
characteristic, regarding various aspects of this head, which was
realized when the head was driven under the above described
condition (9 V/2.7 .mu.sec), and the number of consecutive
recording sheets (A3 sheets of recording paper) through the
printing of which ink was normally ejected, are given in Table
2.
TABLE 2 Emb. 3 Comp. 5 Comp. 6 Comp. 7 Comp. 8 Emb. 4 Emb. 5 Comp.
9 Comp. 10 Sh (.mu.m.sup.2) 576 ditto ditto ditto ditto ditto ditto
ditto ditto (.gtoreq..mu.m .times. 24 .mu.m) So (.mu.m.sup.2) 227
200 314 227 202 225 324 ditto ditto (14.2 .mu.m .times. (15 .mu.m
.times. 15 .mu.m) 14.2 .mu.m) D (.mu.m) .o slashed.7 .o slashed.16
.o slashed.20 .o slashed.17 14.2 15 .o slashed.18 ditto ditto Tn
(.mu.m) 12 12 4 8 12 12 6 6 5 To (.mu.m) 7 9 11 9 9 6 9 9.5 9
Stability good no good no good no good no good good good no good no
good Vol. (10.sup.-18 m.sup.3) 4.5 4.3 4.2 4.1 4.4 4.2 4.8 4.9 4.1
Speed (m/s) 17 15 18 14 15 17.5 16 16 18 Refilling time (.mu.sec)
92 90 920 95 89 86 102 101 158 Properly continued to 110 0.1 0.05
0.2 0.05 110 5 0.8 0.3 D/To 2.4 1.78 1.82 1.89 1.58 2.5 2.0 1.89
2.0
As is evident from Table 2, in Embodiments 3-5, the number of the
consecutive recording sheets, through the printing of which the ink
was normally ejected, was far greater than that in the comparative
examples. This verifies that the present invention successfully
prevented the appearance of unwanted white lines, which would have
appeared if some of the ejection orifices failed to eject ink.
Paying attention to D/To, in Embodiments 3-5, D/To was no less than
2, whereas in Comparative Examples 5-9, it was no more than 2.
Further, in Comparative Examples 5-9, the number of the consecutive
recording sheets, through the printing of which ink was normally
ejected, was small, and also, the unwanted white lines for which
ejection failure is responsible were conspicuous. Thus, D/To is
desired to be no less than 2. In Comparative Example 10, D/To was
2.0, and the frequency of sudden ejection failure was relatively
small. However, in this Comparative Example 10, the height Tn of
each liquid path was 5.0 .mu.m, which was rather low. Therefore,
when the head was driven at a frequency of 10 kHz or higher, the
liquid path could not be refilled fast enough, and therefore, an
image lighter in color than a normal image was printed. In other
words, the number of consecutive recording sheets through the
printing of which ink was normally ejected was small.
As for refilling time, in Comparative Example 7, it was 95 .mu.sec,
which was fast enough to drive the head at the aforementioned
frequency. However, in Comparative Example 6, it was 920 .mu.sec,
which was not fast enough for the driving frequency of 10 kHz. This
is due to the fact that in Comparative Example 6, Tn was 4 .mu.m,
which was rather small. Thus, as long as refilling time is
concerned, the height Tn is desired to be no less than 6 .mu.m.
In Embodiment 4 and Comparative Example 8, the opening of each
ejection orifice was square, which was different from the shapes of
the openings in other embodiments and comparative examples, in
which they were in the form of a true circle. Even in Comparative
Example 6 in which the shape of the opening of the ejection orifice
was truly circular, the sudden ejection failure occurred just as in
the other heads, the openings of the ejection orifices of which
were truly circular. In Embodiment 4, D/To was 2.5, which was
desirable since it was greater than 2. Even though the opening of
the ejection orifice was square, the sudden ejection failure did
not occur. In consideration of the deformation caused by the
pressure generated by bubbles, the thickness To of the orifice
plate is desired to be no less than 4 .mu.m.
Further, in order to accurately evaluate the aforementioned
embodiments and comparative examples in terms of color density and
sudden ejection failure, the liquid ejecting head was activated so
that each sheet of recording paper was "solidly" covered with ink,
and the results were evaluated. Being "solidly" covered means that
the printable area of each sheet of recording paper is covered 100%
by ink dots. In this test, a plurality of A3 size (JIS) sheets of
recording paper were consecutively fed. What was important as a
criterion for evaluating the liquid ejecting heads was whether or
not a liquid ejecting head could normally eject ink to solidly
cover the entirety of at least one of the consecutively fed sheets
of recording paper, with ink. If color density begins to drop, or
sudden ejection failure occurs (head is not acceptable), while a
given liquid ejecting head is used to cover the entirety of a sheet
of recording paper with ink, this head is judged to be impractical,
because in such a case, the printing operation must be interrupted
to carry out a recovery operation or the like, which requires extra
time. In other words, it is essential that it is assured that a
liquid ejecting head can entirely cover at least one sheet (A3
size) of recording paper with ink, without an interruption and
without losing print quality.
In any case, the present invention offers practical solutions, in
terms of liquid ejecting head structure and liquid ejecting method,
to the problems which occur when ink droplets with a volume of no
more than 15.times.10.sup.-15 m.sup.3 are ejected from such a
liquid ejecting head that allows bubbles to become connected to the
atmosphere.
(Miscellaneous)
The present invention brings forth excellent results when applied
to an ink jet based recording head and an ink jet based recording
apparatus, in particular, those which are equipped with means (for
example, electrothermal transducer, laser beam emitting element,
and the like) for generating thermal energy as the energy used for
ejecting ink, and change the state of ink with the use of the
thermal energy. This is due to the fact that according to such an
ink jet system, recording can be made at a high density to produce
highly precise images.
As for the structures and liquid ejection principle for such a
recording head or a recording apparatus, those disclosed in the
specifications of U.S. Pat. Nos. 4,723,129 and 4,740,796 are
desirable. The system disclosed in these patents is compatible with
both the so-called on-demand type and the continuous type, in
particular, the on-demand type for the following reason. That is,
in the on-demand type, each electrothermal transducer is disposed
so that it faces a sheet or a liquid path in which liquid (ink) is
held. In order to eject the liquid, at least one signal, which is
capable of generating a large enough amount of thermal energy to
suddenly increase liquid temperature to a point at which the
so-called film boiling is triggered in the liquid, on the surface
of the electrothermal transducer, is applied to the electrothermal
transducer in accordance with recording data. In other words,
bubbles are formed in the liquid (ink) by driving signals one for
one. As each bubble grows and contracts, the liquid is ejected in
the form of a droplet (at least one droplet) through the opening of
specific ejection orifices correspondent to the recording data. The
driving signal is preferred to be in the form of a pulse, because
the driving signal in the form of a pulse causes a bubble to
instantly and properly grow and contract, in other words, head
response is excellent when the driving signal is in the form of a
pulse.
More specifically, a driving signal such as the driving signal in
the form of a pulse which is disclosed in U.S. Pat. Nos. 4,463,359
and 4,345,262 is suitable. Further, if the condition regarding the
rate of temperature increase at the heat releasing surface of an
electrothermal transducer, which is recorded in the specification
of U.S. Pat. No. 4,313,124 is employed, printing quality can be
further improved.
The present invention is compatible with not only the recording
head structure disclosed in each of the specifications of the
aforementioned patents, in which ejection orifices, liquid path
(right angle liquid path), and electrothermal transducers are
arranged as described above, but also recording heads such as the
recording head structure disclosed in the specifications of U.S.
Pat. Nos. 4,558,333 and 4,459,600, according to which the heat
releasing surface of an electrothermal transducer is located at the
bend of a liquid path. The present invention is also effective when
applied to the recording head structure disclosed in Japanese
Laid-Open Patent Application No. 123670/1984, according to which an
ejection orifice is constituted of a slit shared by a plurality of
electrothermal transducers, or the recording head structure
disclosed in Japanese Laid- Open Patent Application No.
138461/1984, according to which an opening for absorbing pressure
waves generated by thermal energy is placed directly facing the
liquid ejecting section. In other words, the present invention
improves a recording head, such as those described above, in terms
of reliability and efficiency, regardless of its configuration.
Further, the present invention is effectively applicable to a
full-line type recording head, that is, a recording head, the
length of which equals the maximum recording range of a recording
apparatus, that is, the width of the image recordable area of the
largest piece of recording medium which can be accommodated by a
recording apparatus. A full-line recording head may be constituted
of a combination of a plurality of recording heads, the combined
length of which equals the length of the full-line recording head,
or may be formed as a single piece of a long recording head.
The present invention is also effectively applicable to the
aforementioned serial type recording head, which may be in the form
of a fixed type recording head, a chip type recording head, or a
cartridge type recording head. A fixed type recording head is such
a head that is fixed to the main assembly of a recording apparatus.
A chip type recording head is an exchangeable type head, which is
removably installable in the main assembly of a recording
apparatus. As it is installed in the main assembly of a recording
apparatus, it is electrically connected to the main assembly, and
is provided with ink. A cartridge type recording head is such a
head that integrally comprises an ink container.
Providing a recording head with an ejection performance restoring
means, a means for ejecting liquid prior to recording ejection, and
the like means, is desirable since it assures the effectiveness of
the present invention. More specifically, these means are a means
for capping a recording head, a means for cleaning a recording
head, a means for applying positive or negative pressure to a
recording head, a means for heating a recording head or ink prior
to recording ejection, and a means for ejecting ink prior to
recording ejection. A means for heating a recording head or ink
prior to recording ejection may employ an electrothermal transducer
for recording ejection, an electrothermal transducer different from
the one for recording ejection, or a combination of both.
Regarding the recording head type, and the number of recording
heads mounted in a recording apparatus, there is no strict
restriction. For example, the number of recording heads mounted in
a recording apparatus may be only one as it is in the case of a
recording apparatus which prints only in the monochromatic mode, or
may be plural as it is in the case of a recording apparatus which
uses a plurality of inks to print images different in color or
density. In other words, the present invention is very effectively
applicable to not only a recording apparatus equipped with only a
single recording head for the main printing mode, or black mode,
but also a recording apparatus equipped with a plurality of
recording heads, being integral with each other or separate, for
printing in a plurality of recording modes, for example, a
multi-color mode, a full color mode accomplishable by color
mixture, and the like mode inclusive of the monochromatic mode.
In the above description of the embodiments of the present
invention, ink was described as ink in liquid form. However, the
present invention is compatible with such ink that remains solid at
or below the normal room temperature and liquefies above the normal
room temperature. Generally speaking, in an ink jet system, in
order to keep ink viscosity within a range in which ink ejection
remains stable, ink temperature is controlled so that it remains
within a range from no less than 30.degree. C. to no more than
70.degree. C. Thus, the ink to be used with a recording head in
accordance with the present invention may be such ink that
liquefies at the time of recording signal application. Using the
"solid" ink offers additional benefits. For example, the excessive
temperature increase, which will be caused by the excessive energy,
can be prevented by using the excessive energy to change the state
of ink from solid state to liquid state. Ink which remains solid
when left alone, and liquefies as heat is applied to it may be
employed to prevent ink evaporation. In any case, the present
invention is compatible with any of the inks of the above described
types, for example, the solid ink which is liquefied only by the
thermal energy generated by a recording signal, and is ejected in
liquid form, but begins to solidify the moment it reaches recording
medium. One example of such ink is disclosed in Japanese Laid-Open
Patent Application No. 56847/1979 or 71260/1985, according to which
the ink in solid or liquid state is retained in the indentations or
through holes of a sheet of porous material, so that it directly
faces an electrothermal transducer. In terms of compatibility with
this type of ink, a recording head based the aforementioned
film-boiling type system is the best.
As for the field of usage, an ink jet type recording apparatus in
accordance with the present invention can be used as an image
output terminal for an information processing device such as a
computer, a copying apparatus combined with a reader or the like, a
facsimile machine provided with both sending and receiving
functions, or the like.
While the invention has been described with reference to the
structures disclosed herein, it is not confined to the details set
forth and this application is intended to cover such modifications
or changes as may come within the purposes of the improvements or
the scope of the following claims.
* * * * *